Cylinder and manufacturing method therefor

ABSTRACT

A cylinder minimizes the friction between an inner peripheral surface of a cylinder bore and a piston by substantially providing no plating on a portion of the inner peripheral surface of the cylinder bore where a cross sectional configuration of the inner periphery of the cylinder bore is prone to deform and which does not interfere with a predetermined operation performed in cooperation with a piston that reciprocates in slidable contact with the inner peripheral surface of the cylinder bore.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a cylinder of, for example, a two-stroke cycle internal combustion engine or the like, and a manufacturing method for the same.

[0003] 2. Description of the Related Art

[0004] In, for example, a small air-cooled two-stroke cycle internal combustion engine, air pollution by the emission of exhaust gas can be reduced by controlling the friction between a cylinder and a piston that slidably reciprocates in the cylinder to a minimum possible level.

[0005] However, in the assembly process of the internal combustion engine, when the cylinder and a crankcase are joined to each other, that is, when a mounting flange formed at an opening edge of the cylinder is secured to the crankcase by bolts, the bolts are pressed into contact with the mounting flange while it rotates. This tends to cause micron-order torsion that elliptically deforms the cross sectional configuration of the inner periphery of a cylinder bore, which should be perfectly round, by an extremely small amount at the opening edge of the cylinder. Furthermore, the heat generated while the engine is running may cause the opening edge of the cylinder to deform. Such deformation in turn causes a skirt of the piston to partially come into tight contact with the inner peripheral surface of the cylinder bore at the opening edge side when the piston slidably reciprocates, leading to an increase in sliding resistance of the piston. This results in an output loss, adversely affecting the exhaust gas emission ratio to the output of the internal combustion engine.

[0006] Possible solutions to the above problem may include enhancing the strength of the cylinder itself, or suppressing the deformation during the assembly process of the cylinder or the deformation due to the heat by tapering the inner diameter of the cylinder. These solutions are hardly practical because they would result in an excessively heavy cylinder and/or higher cost.

SUMMARY OF THE INVENTION

[0007] Accordingly, it is an object of the present invention to provide a cylinder that allows undue friction between the inner peripheral surface of a cylinder bore and a piston to be suppressed by a simple method, a manufacturing method for the cylinder, a plating method for the cylinder, and a plating apparatus for the cylinder.

[0008] According to one aspect of the present invention, there is provided a cylinder in which the friction between an inner peripheral surface of a cylinder bore and a piston is controlled to a minimum possible level by decreasing the thickness of a plating layer on a portion of the inner peripheral surface of the cylinder bore where a cross sectional configuration thereof is prone to deformation and which does not interfere with a predetermined operation performed in cooperation with the piston that slidably reciprocates in contact with the inner peripheral surface of the cylinder bore.

[0009] In the cylinder, the inner peripheral surface of the cylinder bore is provided with a plating layer having a sufficient thickness for enhancing wear resistance, excluding a non-plated portion which includes a portion provided with a thinner plating layer. The portion of the inner peripheral surface of the cylinder bore where the cross sectional configuration of the cylinder bore easily deforms is substantially made free of plating, thus initially providing a larger clearance relative to the piston owing to the absence of plating. This extra clearance prevents undue friction against the piston from being produced even if the cross sectional configuration of the cylinder bore deforms. Since the non-plated portion of the inner peripheral surface of the cylinder bore does not interfere with the predetermined operation performed by the cylinder and the piston together, the operation of the cylinder will not be affected.

[0010] According to another aspect of the present invention, there is provided a cylinder in which the friction between an inner peripheral surface of a cylinder bore and a piston is controlled to a minimum possible level by substantially eliminating a plating layer on a portion of the inner peripheral surface of the cylinder bore where a cross sectional configuration of the inner periphery thereof is prone to deformation and which does not interfere with a predetermined operation performed in cooperation with the piston that reciprocates in slidable contact with the inner peripheral surface of the cylinder bore.

[0011] According to yet another aspect of the present invention, there is provided a cylinder having an inner peripheral surface of a cylinder bore thereof furnished with plating, substantially excluding a portion of the inner peripheral surface at an opening end of the cylinder.

[0012] The piston is inserted into the cylinder bore from, for example, the opening end portion, and the piston reciprocates in slidable contact with the inner peripheral surface of the cylinder bore. The opening end portion of the cylinder has lower strength than that of the remaining portion because it is open, and the cross sectional configuration of the inner periphery of the cylinder bore is apt to be deformed by various causes. However, the inner peripheral surface of the opening end portion is substantially formed to be a non-plated portion, which also includes a portion provided with a thin layer of plating, thereby initially providing a larger clearance relative to the piston owing to the absence of plating. This extra clearance prevents undue friction against the piston from being produced even if the cross sectional configuration of the cylinder bore inner periphery deforms.

[0013] According to a further aspect of the present invention, there is provided a cylinder which has an inner peripheral surface of a cylinder bore thereof furnished with a plating layer, and slidably receives a piston from an opening end portion thereof, wherein an inner peripheral surface of the opening end portion is substantially free of plating so as to form an annular clearance between the inner peripheral surface and a skirt of the piston that is located lower than the position of a piston ring when the piston is at bottom dead center.

[0014] According to a still further aspect of the present invention, there is provided a manufacturing method for a cylinder having a non-plated portion, including a step of disposing an electrode, which is composed of an insulator and has a partial mask, so that the electrode opposes an inner peripheral surface of a cylinder bore, a step for passing electric current from the electrode to the cylinder, and a step for supplying a plating solution between the electrode and the inner peripheral surface of the cylinder bore.

[0015] According to the manufacturing method in accordance with the present invention, no plating is deposited on the inner peripheral surface of the cylinder bore that matches the position of the mask. Hence, a cylinder having a non-plated portion at a desired location can be easily fabricated by providing the electrode with the mask that matches a portion, which requires no plating, of the inner peripheral surface of the cylinder bore.

[0016] In the manufacturing method for a cylinder having the non-plated portion, the location of the mask provided on the electrode may be the location of a non-plated portion that is formed by the mask and located at an opening end of the inner peripheral surface of the cylinder bore.

[0017] According to a further aspect of the present invention, there is provided a plating method for a cylinder, including a step of disposing an electrode, which is composed of an insulator and has a mask, at a position of an inner peripheral surface of a cylinder bore, where no plating is required, so that the electrode opposes the inner peripheral surface of a cylinder bore, a step for passing electric current from the electrode to the cylinder, and a step for supplying a plating solution between the electrode and the inner surface of the cylinder bore.

[0018] According to still another aspect of the present invention, there is provided a plating apparatus for a cylinder, including a detachable anode that has a mask composed of an insulator and is disposed at the position of a portion of an inner peripheral surface of a cylinder bore, that requires no plating, so that it opposes the inner peripheral surface of the cylinder bore, a cathode electrically connected to the cylinder, and a plating solution supplied between the anode inserted so that it opposes the inner peripheral surface of the cylinder bore and the inner peripheral surface of the cylinder bore.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 is a longitudinal sectional view showing an essential section of a two-stroke cycle internal combustion engine incorporating a cylinder according to an embodiment of the present invention;

[0020]FIG. 2 is an enlarged view of the cylinder of FIG. 1; and

[0021]FIG. 3 is a longitudinal sectional view showing a manufacturing method and a manufacturing apparatus for the cylinder of FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0022] An internal combustion engine 1 shown in FIG. 1 is a small, air-cooled two-stroke cycle gasoline engine of the so-called “schniirle system” type having a displacement of about, for example, 15 to 35 mL (milliliters). This type of engine is small and light-weight, making it ideally suited for use in a driving motor of a portable power working machine, such as a chain saw and a bush cutter.

[0023] The internal combustion engine 1 is equipped with a cylinder 2 according to an embodiment of the present invention disposed so that a head 2 a thereof faces upward. The cylinder 2 has multiple cooling fins 2 b integrally formed on its outer periphery, and a combustion chamber 3 at an upper side therein. The cylinder head 2 a has a spark plug mounting hole 28 formed therethrough, a spark discharging portion 4 a of the spark plug 4 screwed into the spark plug mounting hole 28 being positioned in the combustion chamber 3. An intake port 5 and an exhaust port 6 are formed in the peripheral wall of the cylinder 2 such that they are staggered and oppose each other, the exhaust port 6 being located at a higher level than the intake port 5. The intake port 5 is in communication with a carburetor (not shown), while the exhaust port 6 is in communication with an exhaust muffler (not shown). The cylinder 2 is further provided with a pair of scavenging ports 7 and 7 (only one of them is shown in the drawing) that are formed, facing against each other. The paired scavenging ports 7 and 7 are in communication with a crank chamber 9 via a scavenging passage 8 shown in FIG. 2.

[0024] A piston 10 is disposed such that it may vertically reciprocate in slidable contact with a bore inner peripheral surface 2 c of the cylinder 2, the piston 10 being in contact with the cylinder bore inner peripheral surface 2 c via a piston ring 11 fitted to an upper outer peripheral surface of the piston 10. The piston 10 is guided by the cylinder bore inner peripheral surface 2 c to slidably reciprocate along a vertical axis thereof so as to open and close the intake port 5, the exhaust port 6, and the pair of scavenging ports 7 and 7 of the cylinder 2 in a well-known, standard “piston valve” mode. The reciprocating motion of the piston 10 is converted into a rotary motion of a crankshaft 15, which is equipped with a balance weight 14, through a piston pin 12 and a connecting rod 13, and the shaft output is used as the power for the portable power working machine via, for example, a centrifugal clutch or the like (not shown). The crankshaft 15 is rotatably supported by a crankcase 16 coupled to a lower part of the cylinder 2. The crankcase 16 has the crank chamber 9 therein.

[0025] Referring to FIGS. 1 and 2, the cylinder bore inner peripheral surface 2 c of the cylinder 2 is provided with a plating 17 composed of a metal, such as iron or chromium, for improving the hardness and wear resistance of the inner peripheral surface 2 c. Furthermore, the plating 17 is provided to ease the friction between the bore inner peripheral surface 2 c and the piston ring 11 so as to ensure smooth guiding of the piston ring 11 in addition to hardening the cylinder bore inner peripheral surface 2 c, with which the piston ring 11 constantly slidably contacts, so as to suppress the wear of the bore inner peripheral surface 2 c. In the examples illustrated in FIG. 1 through FIG. 3, the thickness of the plating 17 is exaggerated for the purpose of easier understanding, while the actual plating 17 on the bore inner peripheral surface 2 c has a thickness of about 20 μm to about 25 μm.

[0026] In this embodiment, the plating 17 is not provided on the entire bore inner peripheral surface 2 c; instead, a part of the surface 2 c is not plated and substantially free of plating. The phrase “substantially free of plating” refers to a portion completely free of plating and also a portion provided with a thin layer of plating. The non-plated portion of the bore inner peripheral surface 2 c corresponds to a portion where the cross sectional configuration of the inner periphery of the bore easily deforms and which does not adversely affect a predetermined operation performed by the cylinder 2 and the piston 10 together. More specifically, the non-plated portion is the portion which will not interfere with effective and smooth operation of the internal combustion engine 1.

[0027] As clearly shown in FIG. 2, in the bore inner peripheral surface 2 c in this embodiment, a portion 19 adjacent to a downward opening end 2 d of the cylinder 2 is substantially non-plated. The non-plated portion 19 allows a larger annular clearance 18 than that in a conventional cylinder to be formed between the inner peripheral surface 2 c and the outer peripheral surface of a piston skirt 10 a located at a lower level than the piston ring 11, when the piston 10 reaches the bottom dead center shown in FIG. 1. As a result, even if, for example, a small scale of thermal deformation takes place in the opening end 2 d of the cylinder 2 during the operation of the internal combustion engine 1, causing the bore inner periphery 2 c to be slightly deformed from a perfectly round configuration to an oblong configuration, it will be possible to prevent considerable friction from being produced between the inner peripheral surface 2 c and the outer peripheral surface of the piston skirt 10 a that reciprocates in slidable contact to the inner peripheral surface 2 c. Accordingly, output efficiency of the internal combustion engine 1 will not be deteriorated, thus contributing to a reduction of air pollution by the exhaust gas.

[0028] A preferable extent of the non-plated portion 19 provided on the inner peripheral surface 2 c of the cylinder 2 at the opening end 2 d may be defined as follows. In order to ensure the predetermined operation performed by the cylinder 2 and the piston 10 together, that is, effective operation of the internal combustion engine 1 implemented by compression of air-fuel mixture in this embodiment, the plating 17 is required to be provided over at least the extent of the bore inner peripheral surface 2 c that corresponds to the extent in which the piston ring 11 moves during a reciprocating slide stroke of the piston 10. In the case of a cylinder of the same type as that of the embodiment equipped with an intake port, the plating 17 should be provided on an area lower than a lower edge portion 5 a of the intake port 5 to ensure stable guiding of the piston 10. Experiments have demonstrated that it is preferable to provide the plating 17 at a position about 5 mm below from the lower edge 5 a of the intake port 5, and to provide the non-plated portion 19 thereunder.

[0029] Referring now to FIG. 3, a manufacturing method for the cylinder having the non-plated portion 19 will be described.

[0030] A plating apparatus 21 shown in FIG. 3 is equipped with a fixed jig 22 and a movable jig 23 that can vertically move with respect to the fixed jig 22. The movable jig 23 has a cylinder mounting reference hole 24 that vertically extends. A mounting flange 25 of a cylinder workpiece 20 as an object to be plated, such as an aluminum alloy die cast molding, is applied to the cylinder mounting reference hole 24 of the movable jig 23, and pushed from above by a workpiece fixing and pressing member 26 serving also as a cathode thereby to be fixed on the movable jig 23. The gap between the movable jig 23 and the cylinder workpiece 20 is fluid-tightly sealed by an appropriate sealing member (not shown). The spark plug mounting hole 28 in communication with an internal space 27 of the cylinder workpiece 20 is closed in advance by an appropriate plug 29 formed of a synthetic resin or the like. Furthermore, the intake port 5 and the exhaust port 6 of the cylinder workpiece 20 are hermetically sealed by plugs 30 and 31 that are moved back and forth by, for example, an air cylinder (not shown).

[0031] A cylindrical plating solution supply duct 32 serving also as an anode is secured to the fixed jig 22 such that the passage thereof juts out upward. In a state wherein the movable jig 23 has moved downward together with the cylinder workpiece 20 until it comes in close contact with the fixed jig 22, the plating solution supply duct 32 is positioned concentrically with the internal space 27 of the cylinder workpiece 20, an upper end opening portion 32 a thereof being positioned at the top of the internal space 27 of the cylinder workpiece 20.

[0032] The plating solution supply duct 32 is partly provided with a mask 33 composed of an insulator. Electric current does not flow outside through a portion, which is provided with the mask 33, of the outer peripheral surface of the plating solution supply duct 32 serving as the anode. Hence, the cylinder 2 having the non-plated portion 19 can be easily made by providing the outer peripheral surface of the plating solution supply duct 32, which does not need the plating 17, with the mask 33 so that the mask 33 matches the portion of the bore inner peripheral surface 2 c which does not need the plating 17. In the present embodiment, the mask 33 is provided on the outer peripheral surface of the plating solution supply duct 32 such that the non-plated portion 19 is formed on the inner peripheral surface at the opening end 2 d of the cylinder workpiece 20.

[0033] The mask 33 may be formed of a tubing material that is composed of an appropriate synthetic resin, has a thickness of about 0.8 mm, such as Teflon (a trade name), and exhibits resistance to a plating solution and a plating temperature, and insulating properties. The mask 33 can be easily applied to the plating solution supply duct 32 by making use of, for example, the thermal shrinkability or the like of the synthetic resin.

[0034] The fixed jig 22 has an annular passage 34 in communication with the cylinder mounting reference hole 24 of the movable jig 23, the annular passage 34 being in communication with a passage 35 extending downward in the fixed jig 22. The cylinder mounting reference hole 24 of the movable jig 23 and the annular passage 34 and the downward passage 35 of the fixed jig 22 make up a plating solution drainage passage for returning a plating solution L supplied into the cylinder workpiece 20 back to a reservoir 36 from the internal space 27 of the cylinder workpiece 20.

[0035] The plating solution L, such as an iron plating solution or a chromium plating solution, is forcibly fed from a solution source 38 to the plating solution supply duct 32 by, for example, a pump 37 serving as a plating solution carrying means. The plating solution L flows out from the upper end opening portion 32 a of the plating solution supply duct 32 to fill the gap between the bore inner peripheral surface 2 c and the outer peripheral surface of the plating solution supply duct 32. The positive end of a DC power source 39 is connected to the plating solution supply duct 32, while the negative end of the DC power source 39 is connected to the workpiece fixing and pressing member 26, then electric current is passed from the plating solution supply duct 32 to the bore inner peripheral surface 2 c. This causes the metal in the plating solution L to be deposited and securely electrodeposited on the bore inner peripheral surface 2 c.

[0036] However, the electric current does not flow to the bore inner peripheral surface 2 c through the portion of the outer peripheral surface of the plating solution supply duct 32 that has been provided with the mask 33. This arrangement allows no plating to be electrodeposited on the portion of the bore inner peripheral surface 2 c that corresponds to the portion provided with the mask 33, so that the non-plated portion 19 can be easily formed. Furthermore, in the vicinity of an upper end portion 33 a of the mask 33, the amount of electric current passing from the plating solution supply duct 32 to the bore inner peripheral surface 2 c gradually reduces from top to bottom along the plating solution supply duct 32. Accordingly, the bottom edge portion of the plating 17 electrodeposited onto the bore inner peripheral surface 2 c naturally forms a smooth, downward slope 17 a oriented outward relative to the cylinder workpiece 20, as shown in FIG. 2. This slope 17 a ensures also smooth contact to the outer peripheral surface of the piston 10.

[0037] Moreover, since the plating solution supply duct 32 serving as an electrode is partly provided with the mask 33, a smaller current carrying area is required as compared with a case with no masking by the mask 33. This leads to advantages, including higher electric current efficiency and a shortened plating time.

[0038] The plating solution L supplied into the cylinder workpiece 20 is drained into the reservoir 36 through the plating solution drainage passages 24, 34, and 35.

[0039] After completion of the plating process set forth above, the cylinder workpiece 20 having the non-plated portion 19 is removed from the movable jig 23, and the bore inner peripheral surface 2 c is subjected to honing by a well-known method.

[0040] In the embodiment described above, the outer peripheral surface of the anode used for the plating work is partly masked to substantially form a non-plated portion on a cylinder inner peripheral surface. Alternatively, however, the same advantage can be obtained by reducing the outer diameter of the anode to increase the clearance between the anode and the cylinder inner peripheral surface so as to form a thin plating layer on the corresponding portion (including a substantially non-plated portion).

[0041] The spirit of the present invention is not limited to the cylinder of an internal combustion engine. The present invention can be also applied to the cylinder of other equipment, including an air compressor and a pump. 

What is claimed is:
 1. A cylinder in which the friction between an inner peripheral surface of a cylinder bore and a piston is controlled to a minimum possible level by decreasing the thickness of a plating layer on a portion of an inner peripheral surface of a cylinder bore where a cross sectional configuration of the inner periphery is prone to deformation and which does not interfere with a predetermined operation performed in cooperation with a piston that reciprocates in slidable contact with the inner peripheral surface of the cylinder bore.
 2. A cylinder in which the friction between an inner peripheral surface of a cylinder bore and a piston is controlled to a minimum possible level by substantially eliminating a plating layer on a portion of an inner peripheral surface of a cylinder bore where a cross sectional configuration of the inner periphery is prone to deformation and which does not interfere with a predetermined operation performed in cooperation with a piston that reciprocates in slidable contact with the inner peripheral surface of the cylinder bore.
 3. A cylinder having an inner peripheral surface of a cylinder bore thereof furnished with plating, wherein a portion of the inner peripheral surface at an opening end of the cylinder is substantially formed of a non-plated portion.
 4. A cylinder which has an inner peripheral surface of a cylinder bore thereof furnished with plating, and slidably receives a piston from an opening end portion thereof, wherein a portion of the inner peripheral surface of the opening end portion is substantially formed of a non-plated portion so as to allow an annular clearance between the inner peripheral surface and a skirt of the piston that is located lower than the position of a piston ring when the piston is at bottom dead center.
 5. A manufacturing method for a cylinder having a non-plated portion, comprising: a step of disposing an electrode, which is composed of an insulator and has a partial mask, so that the electrode opposes an inner peripheral surface of a cylinder bore; a step for passing electric current from the electrode to the cylinder; and a step for supplying a plating solution between the electrode and the inner peripheral surface of the cylinder bore.
 6. A manufacturing method for a cylinder having a non-plated portion according to claim 5 , wherein the electrode is provided with masking so as to form a non-plated portion at the opening end of the inner peripheral surface of the cylinder bore.
 7. A plating method for a cylinder, comprising: a step of disposing an electrode, which is composed of an insulator and has a mask, at a position of an inner peripheral surface of a cylinder bore, where no plating is required, such that the electrode opposes the inner peripheral surface of a cylinder bore; a step for passing electric current from the electrode to the cylinder; and a step for supplying a plating solution between the electrode and the inner surface of the cylinder bore.
 8. A plating apparatus for a cylinder, comprising: a detachable anode that has a mask composed of an insulator and is disposed at the position of a portion of an inner peripheral surface of a cylinder bore, the portion requiring no plating, so that it opposes the inner peripheral surface of the cylinder bore; a cathode electrically connected to the cylinder; and a plating solution supplied between the anode, which is inserted so that it opposes the inner peripheral surface of the cylinder bore, and the inner peripheral surface of the cylinder bore. 